Scales of the type which indicates the weight as well as the price of merchandise have a graduation support provided with a multitude of price graduations or indicia, either parallel above each other or arranged side by side, each corresponding to a certain base price of the merchandise, i.e., a certain price per weight unit. In order to project a particular section of the graduation scale, the projection system, i.e. the light source, the condenser and the objective, are movable relative to the graduation scale support. In this manner, a price group to be projected may be selected.
The shifting of the projection system is known types of scales is effected by a hand-operated or motor activated drive which is provided with indexing notches for the various price row positions. Cable driven pulleys are used, for example, in which a cable or belt winds or unwinds around a drum with indexing notches, and this controls the location of the projection system. It is also known to use notched and threaded revolving spindles in connection with an adjustment unit which is coordinated with the projection system. In all known systems the projection unit, that is objective and light source plus condenser, is moved directly by an externally mounted price adjustment mechanism.
In designing a price indicating projection scale an object is to provide the available graduation scale surface with as many price graduations as possible in order to achieve the most accurate detailing of the adjustable base price graduation. This requires high precision of the graduation scale as well as of the moving mechanism of the optical projection system. Presently known projection scales generally enlarge the price graduation scale about 40 times. If the projected graduation scale is to be placed on a focusing screen with a 1-millimeter accuracy, positioning of the optical projection system must be effected with an accuracy of 0.025 mm. Therefore, the adjustment drive must be produced with especially high accuracy and/or precision.
Even the most minute inaccuracies in the adjustment mechanism as well as in the setting mechanism have, therefore, an extremely disadvantageous effect. Thus, a not precisely centered cable roll or even just the influence of the cable thickness can result in an inaccuracy in the positioning of the objective. It is also possible that the price graduations on the graduation scale support are spaced in such a way that they do not precisely correspond with the notched positioning of the objective. Moreover, the spaces between the separate price graduation scales can be imprecise; for example, the distance between the first and last graduation scale increases evenly or unevenly. In such cases, adjustments are extremely difficult and many times impossible so that, in certain cases, large errors in price indication must be accepted.